This invention relates to a process for recycling multicomponent mixed polymer waste. More particularly, this invention relates to a process for recycling multicomponent mixed polymer waste by converting it to yarn.
Recycling waste into useful materials is a growing goal in modern society. Landfills are becoming filled to capacity and new sites are hard to find. Another reason for recycling waste is the global depletion of raw materials needed to make fresh material. Polymer waste, often made from petroleum products, is a fertile area for recycling. Man-made polymers generally do not degrade quickly and petroleum raw materials will eventually be depleted.
Mixed multicomponent polymer waste (polymer mixtures containing two or more polymer components) is a particularly fertile material for recycling. One example of a mixed multicomponent polymer waste system is the bicomponent fiber and products made from bicomponent fibers.
Compared to single component fibers, bicomponent fibers have improved properties for some applications. Bicomponent fibers are made up of two different polymer components which are present as two separate domains. In multicomponent fibers, two or more different polymer components exist as separate domains. Bicomponent and multicomponent fibers are to be distinguished from biconstituent and multiconstituent fibers, respectively, which contain two (biconstituent) or two or more (multiconstituent) different polymers as a mixture rather than as separate domains.
Examples of bicomponent fiber structures include sheath/core, islands-in-the-sea, side-by-side and other configurations such as those disclosed in U.S. Pat. Nos. 3,718,534; 4,370,114; 3,531,368; and 5,162,074; each of the foregoing references being incorporated herein in its entirety.
One popular bicomponent fiber has a nylon 6 (polycaproamide) sheath and a polyethylene terephthalate (PET) core. This type of fiber is especially useful in making non-woven webs since nylon 6 melts at a lower temperature than PET does, which, on heating to at least the melting point of nylon 6, causes spot welding where individual filaments cross. However, certain applications of these bicomponent fibers can generate large quantities of scrap, which are targeted for landfilling. For example, fabric manufacturing processes and automotive carpet backing applications using COLBACK (Registered Trademark of Akzo) yarn (23% nylon 6 sheath, 77% PET core) generate large amounts of yarn scrap, fabric scrap and edge-trim scrap. For example, fabric manufacturing results in over 200 metric tons of scrap per year being sent to landfills, with this amount expected to increase with current and planned production expansions.
Recycled mixed polymer scrap can be quite valuable in the right applications. A particularly desirable option for recycling mixed polymeric waste such as COLBACK yarn and fabric scrap is to convert the waste into yarn for insertion into new fabric.
However, mixed polymer wastes present unique problems for recycling. For example, a drawback to recycling COLBACK yarn scrap and fabric scrap is that although nylon and PET, individually, can be readily recycled, mixtures of nylon and PET are not easily recycled. Recycling of mixed polymeric waste generally involves separation of the polymer components. Separation processes generally involve mechanical separation, chemical separation or a mixture of these approaches. Separating nylon and PET by these conventional processes is not usually economical, however.
Mechanical separation of solids include size and density-based techniques, using such instruments as cyclones and screen classifiers, which require the solids to be physically distinct particles. In recycling COLBACK yarn scrap and fabric scrap, the scrap must be ground to sizes that will readily break into nylon and PET particles. This requires grinding to form particles that are smaller than the diameter of the COLBACK yarn filaments, which have a diameter of about 40 micrometers. Thus, the COLBACK yarn and fabric scrap must be ground to particles fine enough to pass through a 400 mesh screen (37 micrometer size holes). While equipment (such as the Mikro-Atomizer available from Pulverizing Machinery Co.) is available to do this, the process of grinding polymers is difficult and generally expensive, often requiring cryogenic processes.
A chemical separation process for recovering nylon 6 from COLBACK yarn waste is disclosed, for example, in U.S. Pat. No. 5,241,066, which is assigned to BASF Corp. However, a major drawback to using chemical separation processes to separate nylon 6 and PET from mixtures thereof is that there are several solvents that will dissolve nylon 6 but will not dissolve PET. Environmental concerns regarding the use of solvents also makes chemical separation processes undesirable.
Recycling mixed polymer waste can be further hindered if the mixed polymers are incompatible. In general, polymer blends are either compatible or incompatible. Compatible blends, such as certain concentrations of polybutylene terephthalate (PBT) in PET, normally exhibit a single melting point and appear as a homogenous material when viewed under a microscope. Incompatible blends, such as most concentrations of nylon 6 in PET, exhibit multiple melting points and appear as a heterogenous mixture when viewed under a microscope. Incompatible blends often have poor adhesion between the phases.
In the molten state, incompatible polymer blends form liquid dispersions. The behavior of liquid dispersions is fairly well understood for Newtonian fluids but poorly understood for viscoelastic fluids such as polymer melts.
The morphology of these incompatible liquid dispersions presents certain challenges to the spinning and drawing processes. As has been shown, incompatible liquid dispersions such as nylon/PET dispersions display a morphology where one polymer forms globules that are surrounded by the other polymer. The size and shape of these globules affect the properties of the final yarn.
For example, the size and shape of the globules can limit fiber formation from the melt. In general, the globules must be significantly smaller than the spinneret capillary diameter for the mixture to flow in a nearly homogenous manner. Overly large dispersion globules can make fiber spinning impossible. In addition, even if spinning is achieved, large globules can make drawing the spun fibers quite difficult.
The size and shape of the globules are determined by conditions in the melt, where viscous and elastic forces balance the interfacial surface tension between the two immiscible polymers. Globule sizes in melt dispersions are controlled by two dynamic mechanisms: dispersion and coalescence. Dispersion breaks apart globules while coalescence combines them. These mechanisms compete and may reach an equilibrium. Such equilibrium depends on the volumetric concentrations, the local shear rate, type of flow, the interfacial surface tension and the fluid viscosities.
Because smaller globules should make fiber spinning easier, the better processing conditions will encourage globule dispersion and discourage globule coalescence. Globule dispersion is easiest achieved if the ratio of viscosities (globule/matrix) is between about 0.3 and about 1.5 but will not occur if the ratio exceeds 3.0. Dispersion can also be enhanced by decreasing interfacial surface tension and increasing local turbulent shear stress. Compatibilizers or modification of the polymer chemistry (including oxidation) can change the interfacial surface tension. The local shear stress can be increased by many means, such as by increasing the flow rate or by decreasing filtration size and spinneret capillary diameter (though small capillaries may cause other flow problems).
Globule coalescence, which occurs when globules collide and form larger globules, can be suppressed by reducing the volumetric concentration of the globule phase and decreasing the shear rate.
In highly viscous fluids such as polymer melts, dispersion and coalescence may occur slowly, so that equilibrium may not occur during extrusion. If dispersion dominates the process, then longer extrusion residence times should improve the process by reducing globule size. If coalescence dominates the process, shorter residence times should be used to reduce the globule size, thereby improving the process.
During yarn draw-down, which occurs between the die face (or the die swell) and the solidification point of the yarn, the molten polymer flows in extensional flow, during which globule dispersion may continue, along with globule elongation, while coalescence ceases. Elongation stretches the globules into oblong shapes. A globule may break into smaller globules (i.e., disperse) rather than continue to stretch. Both elongating and dispersing the globules should improve the physical properties of the final fiber.
As mentioned above, even if spinning is achieved, drawing a yarn from an incompatible polymer mixture is more difficult than drawing a yarn from a pure polymer or a compatible mixture. In the best case, the blend contains long fibrils that deform along with the matrix polymer. In the worst case, the blend contains large globules that are rigid during drawing. Since the morphology of the yarn depends on the spinning conditions, the tensile behavior of the yarn may vary significantly with variation in spinning. Therefore, processing conditions using incompatible liquid dispersions generally should be designed to yield relatively small globules to make spinning and drawing easier to achieve, resulting in a yarn having improved physical properties.
Several recycling processes use mixed polymers. Reference is made, for example, to Clemson University Professional Development Seminar, "Thermoplastic Waste Reclamation" Feb. 9-10, 1993; Kaminsky, W., "Recycling of Polymeric Materials by Pyrolysis" Makromol Chem , Macromol Symp. 48/49, pp. 381-393 (1991); and Chemical Week, "Honing Technology to Improve Economics", Dec. 18/25, 1991. "Plastic lumber" can be extruded from a mixture of post-consumer polymers, while incineration or "thermal recycling" produces energy from polymer mixtures. Thermal cracking of polymers (pyrolysis) is a developing technology that may be a future option. Most plastics recycling processes, however, require reasonably pure polymers.
Previous efforts, however, to recycle COLBACK yarn scrap and fabric scrap by converting them into yarn have been unsuccessful. The incompatibility of the nylon and the PET is believed to be the reason for the difficulty in forming yarn from the scrap. Even attempts to spin blends of virgin nylon 6 and PET have been unsuccessful.
One object of the present invention, therefore, is to provide a process for recycling multicomponent polymer mixtures.
A further object of the present invention is to provide a process for recycling bicomponent incompatible polymer mixtures.
Another object of the present invention is to provide a process for recycling nylon/PET mixtures.
Still another object of the present invention is to provide a process for recycling COLBACK nylon/PET yarn scrap by converting it into yarn.
These and other objects are achieved in the present invention.